The following abbreviations and terms are herewith defined:
3GPP third generation partnership project
ACK acknowledgment
CQI channel quality indicator(s)
CRS/DRS common reference signal/dedicated reference signal
DL downlink
eNB Node B of an E-UTRAN system
HARQ hybrid automatic repeat request
LTE long term evolution of UTRAN (E-UTRAN or 3.9G)
MCS modulation and coding scheme
NACK negative acknowledgment
Node B base station or similar network access node
OFDM orthogonal frequency division multiplex
P-BCH physical broadcast channel
PDCCH physical downlink control channel
PDSCH physical downlink shared channel
PHICH physical HARQ indicator channel
PRB physical resource block
P-RACH physical radio access channel
P/S-SCH primary/secondary synchronization channel
PUCCH physical uplink control channel
PUSCH physical uplink shared channel
QoS quality of service
RN relay node
SFBC space frequency block coding
UE user equipment (e.g., mobile equipment/station)
UL uplink
UMTS universal mobile telecommunications system
UTRAN UMTS terrestrial radio access network
3GPP is standardizing the long-term evolution (LTE) of the UTRAN radio-access technology which aims to achieve reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator. The current understanding of LTE relevant to these teachings may be seen at 3GPP TR 25.814 (v7.1.0, 2006-09) entitled PHYSICAL LAYER ASPECTS OF EVOLVED UTRA and herein incorporated by reference. Both frequency division duplex (FDD) and time division duplex (TDD) are considered in LTE.
One variation of LTE is termed LTE-Advanced or LTE-A. LTE-Advanced aims to provide significantly enhanced services by means of even higher data rates and lower latencies with reduced cost. Since the new spectrum bands for IMT (international mobile telecommunication, such as detailed at IMT-2000) contain higher frequency bands and LTE-Advanced is aiming at higher data rates, coverage of one eNB is limited due to the high propagation loss and limited energy per bit. Relaying has been proposed in many workshop presentations to enlarge the coverage, to improve the capacity and to improve the cell edge performance. Details of such proposals may be seen, for example, at document R1-082024 entitled “A discussion on some technology components for LTE-Advanced” by Ericsson (3GPP TSG RAN WG1 #53; Kansas City, Mo., USA; May 5-9, 2008); document REV-080006 entitled “Requirements for LTE advanced” by Panasonic (dated Apr. 7, 2008); and document R1-081791 entitled “Technical proposals and considerations for LTE advanced” also by Panasonic (3GPP TSG RAN WG1 #53; Kansas City, Mo., USA; May 5-9, 2008). These are attached to the priority document (U.S. Provisional Patent Application Ser. No. 60/191,485) as respective exhibits A, B and C.
Backward compatibility of LTE-Advanced with LTE is required. For this reason, the design of a transparent relay concept to LTE Release 8 (Rel.8) UEs is attractive, where the node performing relay of data/signaling between the eNB and the UE is essentially transparent to the UE. Though the higher layer relay (referred to at document R1-082024 as self backhauling) will have little impact on the published standard implementation of LTE-Advanced, the relay concept introduces large delay and overhead.
In certain terminal implementations consistent with Rel.8 of LTE for TDD, the channel estimator uses a CRS across subframe boundaries. The channel estimator is reset after each UL subframe. Introduction of relays in the network will need to be compatible with these pre-existing/pre-designed UEs. As relays are not yet specified, this means these Rel.8 UEs have no knowledge of relays in the network. To achieve backward-compatibility with these Rel.8 UEs, the already-designed channel estimation algorithm must be addressed as well as common and shared signaling.
The problem is illustrated by example. Consider the case where a current DL subframe is received by a UE from a RN, and a previous (contiguous) DL subframe is only received by the UE from the eNB. This forms two links: RN to UE and eNB to UE, over the current and previous sub-frames respectively. If that UE is one of the Rel.8 (or similar) ones, it has no way of knowing that it received the two different subframes over two different links. The channel estimator in the UE will interpolate the CRS over both of those sub-frames to estimate the channel for demodulation and decoding. As applied to the PDSCH in the current subframe, this will yield the wrong channel estimate and have a significant impact on PDSCH detection reliability. The problem also exists if, in the current subframe, both the RN and eNB transmit to the UE in a co-operative diversity mode.
Simply said, backward-compatibility with Rel.8 UEs means that relays must not interfere with UEs, since the UEs must be able to receive eNB common signaling for initial cell access (e.g. CRS, P/S-SCH, P-BCH), for neighbor cell monitoring, and also for shared control signaling (e.g. PDCCH, PHICH, PUCCH) so as to acquire parameters that are used for data transmission. This implies that a transparent relay needs to be used, for which one embodiment is set forth at document R1-082470 entitled “Self-backhauling and lower layer signaling” by Ericsson (3GPP TSG RAN SG1 #53 bis; Warsaw, Poland; Jun. 30-Jul. 4, 2008), attached to the above-referenced priority document as Exhibit D.
There are additional issues to consider in making transparent relays compatible with Rel.8 UEs. Asynchronous HARQ was agreed for LTE DL in Rel.8, where the eNB always needs to send PDCCH for retransmission. For the case where relays are used, another issue arises is how the coordination can be done by the eNB and the RN to do concurrent transmission. In the DL, the eNB will send PDCCH and PDSCH in one sub-frame for retransmission when a NACK was received. But prior to that retransmission, the RN has to know which physical resources (time and frequency) are used for retransmission of the packet by the eNB, so that the RN can use the same resources to transmit the same packet concurrently.
Another issue that arises concerns PUCCH coverage. Consider the case where an ACK/NACK response from the UE is erroneously decoded in the eNB and/or in the RN. In such case, the inconsistent interpretation of the UE's ACK/NACK will cause some degradation of QoS. For example, if the Rel.8 UE sends back a NACK, then the eNB may receive/decode an ACK and the RN may receive/decode a NACK. In such a case, the eNB will re-allocate this resource to other users and the RN will do non-adaptive retransmissions. This can easily result in a serious resource collision, and in the presence of that collision the QoS of two users cannot be guaranteed.
DRS for relays was recently proposed by Nortel at document R1-083158 entitled “Some further considerations for Downlink Transparent Relay for LTE-A” (3GPP TSG RAN1 #54; Jeju, Korea; Aug. 18-22, 2008), attached to the above-referenced priority document as Exhibit E.